This invention relates to metal matrix composite materials and, more particularly, to the preparation of such materials by a casting process.
Metal matrix composite materials have gained increasing acceptance as structural materials. Metal matrix composites typically are composed of reinforcing particles such as fibers, grit, powder or the like that are embedded within a metallic matrix. The reinforcement imparts strength, stiffness and other desirable properties to the composite, while the matrix protects the fibers and transfers load within the composite. The two components, matrix and reinforcement, thus cooperate to achieve results improved over what either could provide on its own.
Twenty years ago such materials were little more than laboratory curiosities because of very high production costs and their lack of acceptance by designers. More recently, many applications for such materials have been discovered, and their volume of use has increased. The high cost of manufacturing composite materials remains a problem that slows their further application, and there is an ongoing need for manufacturing methods that produce composite materials of acceptable quality at a price that makes them competitive with more common substitutes such as high-strength alloys.
Unreinforced metallic alloys are usually produced by melting and casting procedures. Melting and casting are not easily applied in the production of reinforced composite materials, because the reinforcement particles may chemically react with the molten metal during melting and casting. Another problem is that the molten metal often does not readily wet the surface of the particles, so that mixtures of the two quickly separate or have poor mechanical properties after casting.
In the past, attempts to produce metal alloy-particulate composites by the addition of particulate material to the molten alloy, followed by casting the resulting mixture have not been particularly successful. It has been postulated that the major difficulty with such an approach is that the most desirable particulates, such as, for example, silicon carbide, are not readily wetted by molten metal alloys and that, because of this, the introduction and retention of the particles in the liquid matrix is extremely difficult, if not impossible.
An ability to prepare such composites by melting and casting would have important technical and economic advantages, and consequently there have been many attempts to produce such composites. It has been suggested that wettability could be achieved by coating the particles with nickel. Another technique has involved promoting wetting of the refractory particles in the melt by saturating the melt with anions of the refractory particles. Another method involves the addition of such elements as lithium, magnesium, silicon, and calcium into the melt prior to the addition of the refractory particles. Still another method involves the addition of particles of silicon carbide to a vigorously agitated, partially solidified slurry of the alloy, maintained at a temperature well below the liquidus temperature of the alloy so that solid metal particles are present. Still another attempt to improve the wettability of the particulates has involved subjecting large particulate materials and fibers in the melt to ion bombardment, mechanical agitation, vacuum, and heat prior to mixing with the molten alloy, in order to remove moisture, oxygen, adsorbed gases, and surface film therefrom.
The fabrication of aluminum alloy-alumina fiber composites in one approach uses a stirrer blade with a paddle type design, the blade being designed to move very close to the walls of the crucible to induce a high shear and create a vortex for introduction of the fibers into the melt. The process also requires a baffle, which is immersed slightly below the surface of the melt with a tilt angle of about 45.degree. in the direction of flow, the function of the baffle being to divert the flow pattern in the melt and to aid in the entrapment of the fibers below the surface of the melt.
In yet another approach, composites such as aluminum-silicon carbide particulate composites are prepared using the vortex method of dispersion of particles. The particles are pre-heated for 60 minutes at 900.degree. C. prior to addition to the melt to aid in their introduction into the melt. The vortex is created by stirring the melt rapidly with a mechanical impeller, which causes a deep vortex to form. The particulate is added through the sides of the vortex in an effort to promote rapid incorporation of the particles into the melt and wetting of the particles by the molten metal. Composites produced by this method tend to have poor bonding of the metal to the particulate, as well as entrapped gas.
In a variation of melting and casting techniques, the reinforcement is provided as a mat of packed material, and the molten metallic alloy is forced under pressure into the spaces remaining. This process, termed infiltration or squeeze casting, produces a composite that is not expensive and difficult to use, since an apparatus specific to each part must be prepared.
All of these prior melting and casting techniques have drawbacks owing largely to the specialized, costly modifications that must be done to the particulate or the melted alloy, in order to accomplish wetting. Moreover, the techniques have not been successful in manufacturing composite materials for large-scale, industrial applications. Instead, the primary method for producing composites having a metal matrix and particulate reinforcement has been powder metallurigical processes which are different from the melting and casting procedures.
In the powder metallurgical processes, carefully sized aluminum powder is mixed with silicon carbide particulate in the presence of an organic solvent. A solvent is necessary to prevent a pyrophoric reaction between the aluminum and oxygen in the air. The mixture is poured into drying trays, and the solvent allowed to evaporate over a period of time. The dry, unconsolidated sheets, which are approximately 0.040 inches thick, are stacked to form a plate of the desired thickness. This fragile stack of sheets is placed into a press and heated to the liquid-solid regime of the matrix, where the metal is slushy in character. The stack is then pressed, consolidating the particles, and forming a solid plate.
In another powder metallurgical process, the silicon carbide particles and aluminum are mixed, as above, but the mixed powder is poured into a cylindrical mold, and consolidated by vacuum hot pressing into a cylindrical billet. Because of the high costs of raw materials, particularly the aluminum powders, and the complexities of the fabrication process, the current costs of the composites discourage their large-scale use in many areas. Both powder processes result in considerable segregation of alloying elements in the metallic matrix material, which is undesirable because of its adverse effect on mechanical and physical properties.
Both of the commercial processes above described result in composites which, while having high moduli and adequate strength, have ductility and formability which are low. The complex superheating and deformation cycle which is required in the above processes produce extensive elemental segregation in the matrix, which decreases ductility and prevents the attainment of maximum matrix and composite strengths. A further problem is the retention of the surface oxide which coated the original aluminum powder particles, this serving to further decrease matrix ductility. It would also appear that the oxide coating prevents the complete wetting of the carbide particles, thus further limiting the ultimate composite properties.
Thus, there exists a continuing need for a fabrication method and apparatus using melting and casting to produce metallic composites containing particulate reinforcement, which are technically acceptable with good properties. The method and apparatus must also be acceptable in that they produce the composite materials relatively inexpensively, both as compared with other methods of manufacturing composites and with methods of manufacturing competitive materials. The present invention fulfills this need, and further provides related advantages.